This invention recovers viscous petroleum from petroleum-containing formations, such as tar sand. There are several major formations that contain petroleum which is too viscous to be recovered by ordinary production methods. Utah has about 26 billion barrels of such viscous petroleum. California has about 220 million barrels. The largest of these formations is in Alberta, Canada, which has almost 1000 billion barrels. The depths of these formations range from surface outcroppings to about 2000 feet.
To date, none of these formations have been commercially produced by an in-situ technology. The only commercial mining operation is in a shallow Athabasca deposit. A second mining project is now about 20% completed. However, there have been many in-situ well-to-well pilots. All of these pilots used thermal recovery after forming communication between injection well and production well. Normally this communication has been formed by introducing a pancake fracture. The drive mechanism has been either steam and combustion (the project at Gregoire Lake) or steam and chemicals (the early work on Lease 13 of the Athabasca deposit). Another means of forming communication has been proposed for the Peace River project, where steam will be injected for several years into an aquifer beneath the tar sand formation. Probably the most active in-situ tar sands pilot has been that at Cold Lake, which uses the huff-and-puff single-well method of steam stimulation. This project has been producing about 4000 barrels per day for several years from about 50 wells.
The most difficult problem in any in-situ tar sand project is forming and keeping communication between injection well and production well. In shallow formations, fracturing to the surface has sometimes interfered with maintaining a satisfactory drive pressure. Problems arise from plugging of the fracture when the heated viscous petroleum cools as it moves toward the production well. The cooled petroleum is almost immobile. For example, its viscosity in the Athabasca formations at reservoir temperature is on the order of 100,000 to 10 million cp. The major problem of forming and keeping communication between injection well and production well is primarily due to the character of the formations. The mobility of fluids may be very low or (as in the Athabasca Tar Sands) almost nil. Thus, the Athabasca Tar Sands are strip mined where the overburden is limited. In some tar sands, hydraulically fracturing has been used to form communication between injection wells and production wells. This has not met with uniform success. The problem is more difficult in the intermediate overburden depths and difficulty in controlling fracture duration, which cannot stand fracturing pressure.
Many methods have been used in trying to recover viscous petroleum from Athabasca tar sand formations. People have tried applying heat to these formations by steam or underground combustion. People have tried using slotted liners positioned in the formations as conduits for hot fluids. However, these methods have been unsuccessful because of the difficulty of forming and keeping communication between the injection well and the production well.
Donald J. Anderson et al. have disclosed a solution to this problem, in their U.S. Pat. No. 3,994,340, which is hereby incorporated by reference to show a HASDrive (Heated Annulus Steam Drive) method. Anderson et al. disclose recovering viscous petroleum from a petroleum-containing formation by providing a steam injection well from the earth's surface through the formation, extending at least one lateral hole from the steam injection well through at least a portion of the formation, forming a flow path (this flow path is commonly called a HAS pipe) in the hole isolated from the formation, circulating a hot fluid through the flow path to reduce the viscosity of the viscous petroleum in the formation adjacent the outside of the HAS pipe to form a communication path for flow of petroleum in the formation, and injecting a driving fluid into the formation through the steam injection well and the communication path to promote flow of petroleum in the formation to production wells penetrating the petroleum-containing formation for recovery from the formation.
The cost of drilling horizontal HAS pipe is high. As an alternative, increasing the pattern width can reduce the horizontal drilling cost per unit area, but as the pattern width increases the areal sweep efficiency decreases.